Expert: John Locke - 4/27/2009

Question
HOW SO CAN A BLOOD TEST BE DONE TO DETERMINE THE FATHER OF A CHILD. HOW IS IT DONE?

Answer
Thanks for using AllExperts. Determining paternity with a blood test is done by techniques that examine the similarity between the DNA of (potential) father and child. DNA is a component within all human cells that carries instructions for how cells should operate; you could think of it as the operating blueprints for cells. The DNA within any cell is identical within one person, so it doesn't matter what kind of cell you obtain--so long as you have a cell, you will be able to obtain DNA. That has implications for specimen collection, as you can see in the website below--you don't necessarily have to have a blood sample taken for paternity testing to be done.

Paternity tests used to consider other factors besides DNA, such as blood type of father and child, but those have been supplanted almost entirely by DNA testing.

A person's DNA complement is inherited equally from their mother and father, each parent supplying one half of the total. The child's DNA as a whole is unique, but it will have close to 50% similarity to both parents (that is, a child's DNA is not identical to their parents', but it has about 50% similarity to each parent because each parent contributed 50% of the overall DNA). DNA paternity testing seeks to show that the father's DNA shares enough similarity with the child's that the chance of such a similarity occurring at random would be basically impossible. DNA samples from the father and child are broken up with enzymes, then compared to see if they are similar. If they share a certain level of similarity, it is considered to be a match.

The techniques that can be used for paternity testing are described on the website below. Note that paternity can be tested for before and after birth. Drawing blood isn't the only option for establishing paternity--in fact, it's usually easier to take a cheek swab from both parent and child.

http://www.americanpregnancy.org/prenataltesting/paternitytesting.html
http://www.geneticprofiles.com/main_files/faq.htm#q2